Sen. Barbara Boxer, who released the 2004 letter, says it shows the company misled regulators on the extent of difference between the old and new equipment.

A letter from a Southern California Edison executive shows the company became concerned about the potential for serious design flaws in replacement steam generators at the San Onofre nuclear plant as early as 2004.

U.S. Sen. Barbara Boxer (D-Calif.), who released the letter Tuesday, said it showed that Edison misled regulators about the extent of differences between the old and new generators and said she will ask the U.S. Justice Department to investigate.

The U.S. Nuclear Regulatory Commission's office of investigations and office of inspector general are already investigating whether there was any wrongdoing by Edison.

The plant was shut down more than a year ago after a steam generator tube sprang a leak and released a small amount of radioactive steam. That led to the discovery that the tubes in the generators — installed in 2010 and 2011 — were wearing down at a rapid rate.

Tube wear has been an ongoing issue in the nuclear industry, but the type and extent of wear found at San Onofre was an anomaly.

Boxer pointed to a section in the November 2004 letter written to steam generator manufacturer Mitsubishi Heavy Industries by Edison Vice President Dwight E. Nunn that says, "although the old and new steam generators will be similar in many respects, they aren't like-for-like replacements."

"That is extremely disturbing that [Edison] would do a self-certification that this was like for like when it wasn't," Boxer said in a phone conference with reporters Tuesday. "I don't have confidence in Southern California Edison, given what I now know."

She was referring to the fact that Edison officials decided they didn't need to request license amendments for most of the design changes in the new steam generators. However, Boxer and others contend that if changes were not "like for like," license amendments were called for.

There has been an ongoing dispute between Edison and opponents of the nuclear facility who contend that the company sidestepped a full review of design changes, including adding hundreds of tubes and making changes to support structures. Edison did request two license amendments relating to the steam generator replacement but not for all of the changes.

Edison said in a statement that "contrary to Sen. Boxer's suggestion," the regulation governing what changes can be made without a license amendment "does NOT require that replacement equipment be 'like for like' or identical to the equipment being replaced."

NRC spokesman Eliot Brenner declined to comment on the issue.

Edison also said the 2004 letter and one written in 2005, which the company released Tuesday, showed "careful oversight" of Mitsubishi's design process by Edison. Both letters were provided to the NRC, the company said.

In the 2004 letter, Nunn noted that original steam generators at San Onofre were among the largest in the U.S. and the replacements would be the largest Mitsubishi had ever built.

He wrote, "It will require Mitsubishi Heavy Industries to evolve a new design beyond that which they currently have available," he wrote.

Nunn also expressed concern that "design flaws could be inadvertently introduced into the steam generator design that will lead to unacceptable consequences (e.g., tube wear and eventually tube plugging). This would be a disastrous outcome for both of us."

The letter then went on to discuss, in detail, areas of concern. One of those was the design of anti-vibration bars intended to prevent tubes from shaking excessively. Nunn wrote that Mitsubishi and Edison "are having difficulty in formulating" a plan. The design of the bars ultimately failed to prevent the tubes from vibrating and knocking against adjacent tubes and support structures.

The NRC is currently weighing a proposal by Edison to restart one of the plant's two units at 70% power.

Edison and Mitsubishi have not yet settled on a long-term repair plan, and Edison has signaled that it may shut the plant down for good if the NRC does not give the go-ahead for the partial restart.